Conventional strain gauges use commutator rings and carbon brushes to transmit signals from rotary strain gauges to non-rotary circuitry. This has given rise to four significant problems, the problem of signal noise due to rubbing contact, loss of signal due to contact lift under vibration conditions, the reduction in life due to wear and the distribution of electrically conductive wear particles through the assembly.
The anchorage of wires in carbon brushes to establish good metal-to-metal carbon electrical contact is well-established art. Conventionally a hole is drilled or formed in one end of a carbon rod forming the brush, and a wire or braid flexible conductor is anchored in Diace by dropping one end of the wire or braid conductor into the hole and packing metal powder, such as copper powder, into the hole around the wire. The metal powder is then tamped into place, followed by an optional sintering step. The anchorages are reasonably firm and secure, despite the fact that the carbon rod forming the brush is under a permanent potential fracture stress. Modern trends towards maintenance-free equipment mean that the performance specifications for carbon brushes are rising, and even the relatively low incidence of failure in such brushes historically accepted by the industry are now considered as unacceptable failure rates in some applications. One such application is in torque transducers for which there is a strong tendency to impose a performance specification that guarantees error free operation for life. Any such equipment, therefore, utilising traditional solid rod carbon brushes therefore falls short of that specification.
Torque measurement by strain gauges attached to torque transmitting shafts has been subject to signal noise as a result of slipring-brush contact variation. The noise can be particularly obtrusive where impact or pulse torque is applied, suddenly placing the shaft for short moment under considerable lateral vibration.
Our published patent specification EP 232606 describes a novel torque transducer in which the conventional carbon brushes for picking the electrical signals from the metal commutator sliprings are replaced by flexible strips of carbon material laminated to flexible packing strips, passing partially around metal sliprings and tensioned against the sliprings. The carbon material on the flexible laminated strips is in brushing contact with the metal sliprings, and the long arc of contact effectively removes substantially all signal noise. It is desirable that the arc of contact is in excess of 180 degrees. Moreover the problem of establishing a good carbon-to-metal electrical contact between the flexible laminated strips is solved by simply clamping each flexible laminated strip in an electrically conductive clamping member.
The above solution is highly satisfactory for most applications, but requires regular brush replacement. The constant slipping of the metal sliprings against the thin carbon layer of the flexible laminated strips causes the carbon to become worn away, and when the carbon surface is no longer contiguous, signal noise again becomes a problem. The unit cannot therefore be sold as a guaranteed service-free unit.
It is therefore highly desirable to design a torque transducer which has the low signal noise of EP 232606 but which is not subject to the same problems of wear, and which can therefore be guaranteed for a service-free life.